Web Animations: Use StringKeyframes for element.animate()

This patch refactors element.animate()'s use of KeyframeEffectModel to
be based on String keyframes instead of AnimatableValue keyframes.
element.animate() is no longer responsible for creating snapshots of
the keyframe values, this logic is now pushed into
StringPropertySpecificKeyframe::createInterpolation() where we can work
on removing the use of AnimatableValues in favour of more primitive
InterpolableValues.

Review URL: https://codereview.chromium.org/194733002

git-svn-id: svn://svn.chromium.org/blink/trunk@170943 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations API: Remove unused definitions that moved from Animation to TimingInput

Functionality was moved in r168575[1] but these unused definitions remained.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=168575

Review URL: https://codereview.chromium.org/227353002

git-svn-id: svn://svn.chromium.org/blink/trunk@170944 bbb929c8-8fbe-4397-9dbb-9b2b20218538